1. Field of the Invention
The present invention relates to optical transmission systems and, more specifically, to a system of optically transmitting a frequency-multiplexed signal with a plurality of signals frequency-multiplexed therein.
2. Description of the Background Art
FIG. 14 is a block diagram showing an example of configuration of a conventional optical transmission system for a frequency-multiplexed signal. This optical transmission system is hereinafter referred to as a first background art. In FIG. 14, a multiplexer 1400 to which a plurality of signals having carriers with predetermined different frequencies are supplied; a modulator 1408 to which a signal outputted from the multiplexer 1400 is supplied; an optical transmitter 1404 to which a signal outputted from the modulator 1408 is supplied; an optical receiver 1405 for receiving an optical signal sent from the optical transmitter 1404; and a demodulator 1409 to which a signal outputted from the optical receiver 1405 is supplied.
The operation of the above optical transmission system in the first background art is now described. The multiplexer 1400 frequency-multiplexes the plurality of signals having the carriers with different predetermined frequencies, and outputs a multiplexed signal to the modulator 1408. The modulator 1408 modulates the frequency-multiplexed signal to produce a predetermined modulated signal, and outputs the same to the optical transmitter 1404. For such modulation, a frequency modulation (FM) scheme is used, for example. The optical transmitter 1404 converts the modulated signal into an optical signal, and sends the optical signal to an optical transmission path or the like(not shown). The optical receiver 1405 converts the optical signal received through the optical transmission path into the original electrical modulated signal, and outputs the same to the demodulator 1409. The demodulator 1409 demodulates the modulated signal to reproduce the original frequency-multiplexed signal.
The above described first background art is disclosed in detail in “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems”, IOOC' 95, Technical Digest, Vol. 5 PD2-7, which is incorporated herein by reference. In this optical transmission system, a frequency-multiplexed signal is modulated to be an FM modulated signal, and, after optical transmission, demodulated to be reproduced. This can improve SNR (signal-to-noise ratio) of the demodulated frequency-multiplexed signal by using FM gain in FM transmission. Therefore, multi-channel signals can be transmitted with high quality via a single optical fiber.
FIG. 15 is a block diagram showing another example of configuration of the conventional optical transmission system, which is hereinafter referred to as a second background art. In FIG. 15, the optical transmission system includes a multiplexer 1500 to which a plurality of signals having carriers with different frequencies are supplied; an optical transmitter 1504 to which a signal outputted from the multiplexer 1500 is supplied; and an optical receiver 1505 for receiving an optical signal from the optical transmitter 1504.
The operation of the above optical transmission system in the second background art is now described. The multiplexer 1500 frequency-multiplexes a plurality of signals having carriers with predetermined different frequencies, and outputs a frequency-multiplexed signal to the optical transmitter 1504. The optical transmitter 1504 converts the frequency-multiplexed signal into an optical signal, and sends the same to an optical transmission path or the like. The optical receiver 1505 converts the optical signal received via the optical transmission path into the original electrical frequency-multiplexed signal.
In the second background art, a frequency-multiplexed signal is directly converted into an optical modulated signal for optical transmission. Therefore, unlike the first background art, SNR improvement with FM gain cannot be achieved in this transmission system. However, multi-channel signals can be transmitted with simpler structure and low cost via a single optical fiber.
The conventional optical transmission system as described in the first background art can achieve multi-channel signal transmission using an optical fiber with high quality.
However, in the first background art, the following problems may arise due to the characteristics of the frequency-multiplexed signal. The frequency-multiplexed signal is generated by frequency-multiplexing a plurality of signals with different frequencies and phases. The instantaneous amplitude of such frequency-modulated signal is not constant and varies with time. FIG. 16 is a graph illustrating instantaneous amplitude variations on a time axis. As shown in FIG. 16, when a plurality of signals with different frequencies and phases are frequency-multiplexed, coincidences of their peaks in amplitude cause an instantaneous amplitude increase of the frequency-multiplexed signal at a certain time.
In the optical transmission system as described in the first background art, the frequency spectrum width of the modulated signal is determined in FM modulation according to the amplitude of the frequency-multiplexed signal. Therefore, as the amplitude of the frequency-multiplexed signal is instantaneously increased, the corresponding spectrum width of the modulated signal is instantaneously increased.
Furthermore, in an output part of the demodulator 1409 in the first background art, part of the modulated signal components may remain together with the demodulated signal due to circuitry configuration. Such component is herein called a residual modulated signal. It is known that part of the frequency spectrum of the residual modulated signal causes deterioration in the quality of the demodulated signal, which is disclosed, for example, in “CNR characteristics of optical video transmission system using broadband FM modulation scheme”, Fuse et al., Institute of Electronics, Information and Communication Engineers Papers, B-1, Vol. J81-B-1, No. 9, August, 1998.
FIG. 17 is a graph illustrating the relation between the residual modulated signal and the demodulated signal on a frequency axis. Similar to variations in the frequency spectrum width of the frequency-multiplexed signal, variations in the frequency spectrum width of the residual modulation signal correspond to variations in the amplitude of the frequency-multiplexed signal. Therefore, an instantaneous amplitude increase of the frequency-multiplexed signal causes an instantaneous increase in the spectrum width of the residual modulated signal, further interfering with the frequency band of the demodulated signal. As a result, the quality of the demodulated signal deteriorates instantaneously.
In addition to the above, such instantaneous amplitude increase in the frequency-multiplexed signal causes the following problems. That is, such instantaneous increase also causes an instantaneous increase in the corresponding spectrum width of the modulated signal. Then, the modulated signal with its instantaneous spectrum width increased is transmitted through a transmission path such as an optical fiber.
In general, the frequency band of the signal that can be transmitted with good quality through the transmission path is predetermined by design. Therefore, if the instantaneous spectrum width of the modulated signal to be transmitted increases over the predetermined bandwidth predetermined by design, the increased part is clipped or distorted. As a result, the quality of the demodulated signal deteriorates instantaneously.
On the other hand, the optical transmission system as described in the second background art in which the frequency-multiplexed signal is directly converted into an optical modulated signal for optical transmission, multi-channel signal transmission using an optical fiber can be achieved with low cost.
However, also in the optical transmission system of the second background art, the following problems may occur due to the characteristics of the frequency-multiplexed signal, like the first background art.
The optical transmitter 1504 in the second background art generally uses a scheme called direct modulation. In the direct modulation scheme, a current injected to a light source such as a semiconductor laser is modulated with a modulating signal to be an optical intensity modulated signal.
FIG. 18 is a graph illustrating characteristics of input current to output optical intensity in the light source such as a laser device included in the optical transmitter. As shown in FIG. 18, when the input current falls down a threshold value (Ith), the output light power waveform is distorted with the part below the threshold value clipped. Therefore, if the frequency-multiplexed signal is used as the input current signal, its instantaneous increase in amplitude causes distortion in the waveform of the transmission signal, and therefore the quality thereof deteriorates instantaneously.
As described above, in the optical transmission systems as shown in the first and second background arts, an instantaneous amplitude increase, which characterizes the frequency-multiplexed signal, causes deterioration in the quality of the transmission signal.